1. Field of the Invention
The present invention relates to a plasma spray apparatus for spraying powdery or gaseous material, comprising an indirect plasmatron for creating an elongated plasma torch and means for axially feeding the powdery or gaseous material into the plasma torch. Such a plasmatron comprises a cathode assembly, an annular anode member located distantly from the cathode assembly and a plasma channel extending from the cathode assembly to the anode member.
The plasma channel is delimited by the annular anode member as well as by a plurality of annular neutrode members which are electrically insulated from each other.
For spraying e.g. powdery material in a molten state onto a substrate surface, such plasma spray apparatusses are well known in the art which make use of an indirect plasmatron, i.e. an apparatus for creating a plasma with a plasma torch escaping from a nozzle-like element which plasma torch is electrically not current conductive. Usually, the plasma is created by means of a torch and guided through a plasma channel to an outlet nozzle. Thereby, an important difference exists between an apparatus with a short plasma torch and an apparatus with an elongated plasma torch.
2. Prior Art
In a major portion of all plasma spray apparatusses which are commercially used in these days, the plasma torch is created by means of a high current arc discharge between a pin-shaped cathode member and a hollow cylinder anode member. Thereby, the coating material which has to be molten and axially accelerated, e.g. powdery material like metallic or ceramic powder, is introduced into the plasma torch from the side in the region of the anode member which simultaneously forms the outlet opening of the outlet nozzle. Such proceeding of powder feeding, however, is not advantageous as the powder particles are subjected to a different treatment in the plasma torch, depending on their size and on the velocity with which they are introduced into the plasma torch. For instance, big powder particles pass the plasma torch and are not molten. The result is that the coating material is not fully used for coating a substrate surface and that the quality of the surface to be coated is of inferior quality. Furthermore, the complex relations between the operating parameters render the optimization of the plasma spray process much more complicate. Mainly the disturbance of the plasma torch by the radially fed carrier gas which PG,5 is necessary for feeding the coating powder into the plasma torch is very disadvantageous.
The European Patent Application Nr. 0 249 238 discloses a plasma generating system in which the supply of the material to be sprayed onto the surface of a substrate is accomplished in axial direction. Particularly, there is provided a tube which enters the apparatus in radial direction through the side wall of a nozzle which is positioned in front of the anode, continues to the center of this nozzle and is bent into a direction corresponding to the axis of the nozzle. However, the arrangement of a supply tube in the center of the plasma torch leads to difficulties because the supply tube and the plasma torch influence each other in a disadvantageous manner. This means, on the one hand, that the flow of the plasma torch is hindered by the provision of the supply tube, and, on the other hand, the supply tube situated in the center of the plasma torch is exposed to an extremely high thermal load.
As far as the energy balance is concerned, the plasma spray devices known in the prior art have a very bad efficiency. One important reason is that only that part of the energy is used for melting the coating material which is present at the end of the plasma torch where it merges into the free plasma flow if the coating material is fed into the plasma torch in the region of the anode member. In fact, a major part of the supplied energy is lost within the plasma channel because the walls of the plasma channel are heated by the plasma torch; thus, this energy is lost for melting the coating material.
These facts are especially true for plasmatrons with an elongated plasma torch. According to the already mentioned EP 0 249 238, such a plasmatron comprises an elongate plasma channel extending from a cathode to an anode. The plasma channel is defined by the interior of a plurality of annular neutrodes which are electrically insulated from each other. An elongated plasma torch, in fact, can develop a higher thermal energy than a short plasma torch, is subjected, on the other hand, to more pronounced cooling along its way through the long, relatively narrow plasma channel.
Under these circumstances, the result is that all efforts to obtain an energy concentration in the free plasma which is as high as possible, i.e. in that region of the plasma where the coating material is fed, cannot lead to a substantive improvement of the efficiency due to the reasons discussed hereinabove.
However, some suggestions have been made in the prior art to design plasma spray apparatusses such that their specifications are improved. Particularly, it has been suggested to feed the coating material in the cathode side end of the plasma channel.
The German Utility Model Nr. 1,932,150 discloses a plasma spray apparatus of this kind for spraying powdery material, comprising an indirect plasmatron operating with a short plasma torch. A hollow cathode member cooperates with an anode member which also is of hollow design in the kind of an outlet nozzle. The cathode member and the anode member are coaxially arranged and the cathode member extends into the interior of the annular anode member. The hollow cathode member simultaneously serves as a supply tube for the coating material which, in this manner, is introduced into the space where the plasma torch is created. The plasma gas is fed into the space where the plasma torch is created through an annular gap between the cathode member and the anode member and, therefrom, into the anode member nozzle whereby the plasma torch is narrowed. A major disadvantage of this design is that very high currents have to been used to create the plasma torch and, consequently, the useful operating life of the apparatus is quite low.
Furthermore, it must be mentioned that the mean sojourn time of the coating material escaping from the hollow cathode member in the space where the plasma torch is created is relatively short with the result that the particles of the coating material during its presence in this space can absorb only a small amount of thermal energy, especially because the plasma torch is created initially at the edge of the hollow cathode member and not in the axis in which the coating material is fed. It may be an advantage, under these circumstances, that the powder particles are not completely molten before they escape out of the anode nozzle and, therefore, cannot deposit at the wall of the anode nozzle. However, to completely melt the powder particles and to accelerate them, the paramount portion of energy must be delivered by the free plasma flow which has left the anode nozzle.
The application of a hollow cathode member in a plasmatron with an elongated plasma torch, however, presents pronounced technical difficulties, particularly if the plasmatron is operated at high current levels. The reason is that the plasma torch usually is generated at a locally limited point of the cathode with the result that the related cathode part is thermally overloaded and that the cathode wears out very rapidly. It is possible to electromagnetically rotate the point of origin of the plasma torch to render this effects less severe, or to mechanically adjust the cathode as disclosed in the above mentioned EP 0 249 238 to compensate for wear of the cathode, but both methods are quite complicated and require an increased constructional effort and expense.